Serial killer Levi Bellfield has made a “very detailed” confession to the brutal hammer murders of a mother and daughter, lawyers acting for a man twice convicted of the killings claim.

Michael Stone is serving three life sentences for bludgeoning Lin Russell and her daughter Megan, six, to death in an attack near Chillenden in Kent in 1996 in which her other daughter, Josie, nine, suffered severe head injuries and the family’s dog, Lucy, was killed.

But now lawyers representing Stone, 57, who has always maintained his innocence, said Bellfield allegedly made a confession, which included details not made public, to another prisoner.

His sister Barbara Stone hailed the claims as her brother’s “biggest hope” in his protracted legal battle to try and clear his name.

But Bellfield, who is serving two whole life prison sentences for murdering two women and schoolgirl Milly Dowler, 13, has denied making any such confession, saying in audio from behind bars that his accuser is a “known fantasist”.

At a press conference on Wednesday at 33 Bedford Row Chambers, in central London, Stone’s solicitor Paul Bacon said: “We have now received evidence of a full confession by Levi Bellfield to the Russell murders.

“In the confession, Bellfield describes how he came across Lin Russell and her two children, how he attacked them with a hammer and his motivation for the killing.”

Mr Bacon added that they had been contacted by an “independent witness”, who claims to have seen Bellfield at the scene and had made a statement to Kent Police a month after the killings, and also had forensic evidence which they say “corroborates” the confession.

Levi Bellfield (Metropolitan Police/PA)

He said: “We know that Levi Bellfield will deny the confession and will make up counter allegations but we also know that Bellfield is very manipulative.

“The confession, as I have said, is very detailed and he has said a number of matters which are not in the public domain.”

Barrister Mark McDonald added later that Bellfield allegedly made the confession to a serious sex offender at HMP Frankland in Durham following a BBC2 documentary on the Chillenden murders which aired in May this year.

Bellfield is said to have been anxious about the coverage and his confession allegedly includes diagrams of the murder scene and where the bodies were left.

Mr Bacon said the “informant” came forward to his own solicitor, who reported it to Stone’s team, and he would be willing to give evidence.

Lin Russell with the family's dogs (Kent Police/PA)

He also said there was concern over Kent Police’s investigation, while Mr McDonald said the force were “unenthusiastic” when passed the new evidence.

The Criminal Cases Review Commission (CCRC), which had previously dismissed an appeal in 2010, has confirmed it had received an application from Stone’s lawyers in August and the case is under investigation.

Stone was convicted for a second time in 2001 after Court of Appeal judges quashed his first conviction in 1998.

There was no forensic evidence against Stone and he was convicted on the evidence of Damien Daley, who admitted in the witness box that he had lied about his drug-taking exploits at the first trial in 1998.

Daley said Stone confessed to the hammer murders through a heating pipe into the next cell at Canterbury Prison, something Stone maintains was a lie.

He was told in 2006 that he must spend a minimum of 25 years behind bars over the attack on Dr Russell, 45, and her daughters as they walked home from a swimming gala along a country lane in July 1996.

Josie survived her injuries and went on to carve out a new life as an artist, growing up with her father, Dr Shaun Russell, in Wales. Speaking from his home in Wales, Dr Russell said he would not be commenting on the matter.

In November last year, the Metropolitan Police closed a probe into serious crimes allegedly involving Bellfield, saying all lines of inquiry had been “exhausted” and officers had found no evidence linking him to cases for which he had not already been convicted.

Ms Stone urged the CCRC to “act quickly” to refer the case to the Court of Appeal. She said: “Mick’s been in prison for 20 years and that’s 20 years too long for somebody who has not committed a crime.”

She said that, while both Bellfield and her brother were in the same prison, they had only ever had “minimal” contact and “nothing recently”, as they are housed on separate wings. “Their paths don’t cross,” she said.

Bellfield, in audio obtained by the Mirror, claimed his accuser, who he named as serial rapist Richard Baker, was a “fantasist, manipulator and compulsive liar”.

He continues: “To hear such rumours about him was to be expected, given his character. He is a known fantasist and his latest endeavours to pervert the course of justice are an example of his character which seeks attention and publicity.”

He added: “In my view, such vile, untrue allegations do not warrant a response.”

Bellfield, who now calls himself Yusuf Rahim, was jailed in 2011 after being found guilty of abducting and killing Milly Dowler following a trial at the Old Bailey.